If this time you always hear this sound every login at ubuntu, may you will never find it in Ubuntu 12.04 Precise Pangolin, because it's will disabled by default - confirmed by jeremy bicha

the Ubuntu startup sound which have been used for 5 years will be disable by default in Precise Pangolin, on the latest Ubuntu 12.04 development update news, a new version of libcanberra has been upload with startup sound which is disabled.

Ubuntu system sound setting

One thing must be stressed is this sound is disable but not removed, if you want to hear it again, you can enable it in System Setting > Sound (* see screenshot above).